U.S. Pat. No. 3,596,071 discloses and claims a method and apparatus for the control of a coater and a dryer of a material treater wherein variables such as moisture content, resin solids concentration, percent resin and percent volatiles are measured and/or computed according to various mathematical formulae. The computed values are then used to control the rate of resins application and the amount of drying of the treated material such as by varying the heat input to the dryer by means of adjustable dampers. The controls effected by the patentee are not capable of optimizing line speed with the constraints of maximum web temperature and maximum specification line speed because no provision is set forth therein wherein the line speed is adjusted in conjunction with variations in the air velocity impinging on the web.
U.S. Pat. No. 3,845,287, assigned to Formica Corporation, overcame the difficulties exhibited by the prior art control procedures and apparatus in the treatment of material webs. This invention permitted higher line speeds to be obtained for a given material treated, while maintaining the desired drying level and preventing the desired maximum web temperature from being exceeded. The desired results are obtained by the use of a computer which executes the logic required to increase treater line speed, by changing dryer zone temperatures.
Although this patent achieves improved line speeds and dryer control, as measured by % VA or other control formulae, the slow response of the dryer temperatures to set point changes result in suboptimal line speeds and dryer control via % VA (or other mathematical formulae).
The present invention overcomes the slow dryer temperature response problem by using a combination control method and apparatus. The drying rate can be controlled by controlling the air velocity of the dryer of the material treater as it impinges on the material web. The dryer control formulae (% VA or other) can be controlled precisely and instantaneously by changing the dryer supply fan RPMs, varying damper settings etc. to change the air velocity and air quantity thus effecting a change in drying rate, hence % VA, while maintaining line speed at a constant, maximum desirable value.
Other benefits accure from running at a constant, maximum line speed, in addition to increased productivity. Some of these are:
1. No interactions between the coater and dryer line speed changes. Line speed changes upset coating operations, necessitating feed forward control schemes for coater/line speed interactions, resulting in off-spec coat weight control and increased control system complexity.
2. No resetup of the stacker/cutter system required. Changes in speed often necessitate repositioning of stacker belts to optimize stacker operations. Stacker jams can result, causing equipment down time, hence lost production.